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References
D., CHAUVET, M., VIALLET, J.E., and CHAWKI, M.J.: 'First
tunable narrowband 1 . 5 5 optical
~
drop filter using a dynamic
photorefractive grating in iron doped indium phosphide', Electron.
Lett., 1994, 30, (22), pp. 1883-1884
HERVE, D.,
CHAUVET, M.,
VIALLET, J.E.,
and COQUILLE, R :
'Polarization independent tunable narrowband 1 . 5 5 grating
~
filter in photorefractive iron doped indium phosphide'. Proceedings
of IEEE Lasers and Electro-Optics Society Annual Meeting, San
Francisco, CA, 1995
YAMADA, M., and SAKUDA, K.: 'Analysis of almost-periodic
distributed feedback slab waveguides via fundamental matrix
approach', Appl. Opt., 1987, 26, (16), pp. 34743478
French patent 951 1469
DELORME, F., SLEMPKES, s., ALIBERT, G., ROSE, B., and BRANDON, J.:
'Butt-jointed DBR laser with 15nm tunability grown in three
MOVPE steps', Electron. Lett., 1995, 31, (15), pp. 1241c1245
HERVE,
Optical subcarrier multiplexed system using
microwave-optical mixing
Lu Chao
Indexing terms: Subcarrier multiplexing, Optical communication
~
Optical SCM systems using microwave-optical mixing are
proposed in the Letter. Narrowband optical resonant receivers
instead of wideband optical receivers employed in conventional
SCM systems can be used in this scheme. Performance
evaluations for both intensity modulated and external modulated
SCM systemsusing microwave optical mixing are discussed.
Introduction: In a conventional optical subcarrier multiplexing system (SCM), a receiver with a bandwidth greater than the highest
subcarrier frequency is used. This requires a wideband photodiode
and a wideband preamplifier with low equivalent input noise current, which is difficult to design especially when the subcarrier is
in the tens of gigahertz range. To overcome this problem several
schemes have been proposed. These include a coherent SCM system [I] and an optical prefdtering SCM system [2]. However, complicated control schemes are needed to offset lock two optical
carriers or to lock the optical fdter passband to the selected optical-microwave frequency. An optical-microwave mixer based on a
Mach-Zehnder modulator was proposed in [3] for sensor applications to increase the dynamic range. It offers a good way to convert high frequcucy microwave signals down to lower frequency
signals using optical means. This optical-microwave mixer can be
applied to SCM systems as proposed in this Letter. To evaluate
the performance of the systems, both direct modulated and external modulated SCM systems with microwave-optical mixing are
discussed in this Letter.
a
laser
source
M-2
modulator
'ne,,'
local osci Ilator
Direct modulation; b external modulation
Theory: The proposed SCM system is shown in Fig. 1. At the
transmitting end subcarrier multiplexed multichannel signals can
either be used to directly modulate the injecting current of a laser
diode, or a Mach-Zehnder external modulator can be used to
change the laser diode output power according to the SCM multi-
ELECTRONICS LETTERS
25th April 1996
N
where Le is tb: excess loss caused by optical fibre and connectors,
a(t)is an electrical modulating signal, Pra,yer
is the laser diode output power, mi is the modulation index for each subcarrier channel,
N is the number of subcarrier channels andf; are subcarrier frequencies. Oth'Er parameters follow the same definition as in [3].
We can expand eqns. I and 2 in a series of Bessel functions.
Following ;I similar method as given in [l], the carriednoise
ratio at the output of the bandpass filter is given by
C -~
R
( $ R P ~ T D L mi
, . J~ (
El2
2e(~lZP~,,TDL,)Bn~i,,)2L~BPS(a~~~~~L,)2RINBBP
(3)
for the direct imodulation system, or
C -R
( E P ~ T L,
; J~
) .r1(
(E 2)
J,"
( 2))
2 e ($ RPL, ?'D Le)BB P
BB &$
RPh??T D
-
RINBB F b % z d
(4)
for the extern(x1modulated system, where R is the responsivity of
the photodiode, <icy> is the equivalent input noise current density
of the optical receiver, BBpis the bandpass filter bandwidth, RIN
is the relative intensity noise, and a$ is the mixing product term
generated by JfLo and 3rd-order intcrmodulation product terms of
the first external modulator, while 02, is the mixing product terms
generated by ?fL,and higher-order product terms generated by the
frst external modulator. We have
a?zd -
(5)
where K3 is the number of IMPS and is bounded by 3 P / 8 [l]. By
careful examination we can see that 02, = 0 if 2hF2 f,,, - f,,,,
where fmSxand f,,, are the maximum and minimum frequency of
the subcarrier channels and f i F = f; -Lo. This requirement is
necessary in order to avoid the image frequency problem.
Numerical results and discussion: To evaluate the performance of
demodulator
Fig. 1 SCM system using optical-microwave mixer
a
plexed signals. At the receiving end, a microwave-optical mixer is
used to mix the incoming signal with a microwave tunable LO.
This will convert the required channel frequency to a predetermined IF frequency. It can be followed by an optional fibre amplifier to boost the received signal further before a narrowband
resonant optical receiver is used to convert the optical signal to an
electrical signal. An electrical bandpass filter can then be used to
reject unwanted signals and a demodulator will be used to recover
the original transmitted signal. The signals at the outputs of the
Mach-Zehnder mixer for direct and external modulated SCM are
given by:
Vol. 32
the proposed systems, the output carrierhoke ratio (CNR) of a
typical system is studied. A solid-state laser with Pi,,, = 15dBm is
used for the external modulated system, while a DFB laser with
Pi,,,, = 3dBm is used for the direct modulated system. The modulator insertioin loss T, is 4dB. Other parameters are: BBp =
60MHz, R = 0.7, RIN = -155dB/Hz, <ieq> = 3pA/dHz, Le =
15dB and V, = V,,, = 9V. There are all together 16 subcarrier
channels, whiish are equally spaced between 4 and 5.5GHz. j& is
fmed at 1GHz. The CNFUmodulation index is shown in Fig. 2 for
different excess loss levels. Here m, for external modulation is
defined as nV/V,. For a direct modulation system, the CNR
increases with the modulation index without considering clipping
distortion. For an external modulation system there is an optimum modulation index which increases with excess loss levels.
No. 9
839
Polarisation-independent all-optical
demultiplexing up to 200Gbit/s using fourwave mixing in a semiconductor laser
amplifier
t
U
0
.c
_
T. Morioka, H. Takara, S. Kawanishi, K . Uchiyama
a n d M. Saruwatari
0
L
aJ
.c
0
0
+-
Indexing terms: Multiwave mixing, Demultiplexing, Semiconductor
lasers
L
a,
L
Polarisation-independent all-optical time-division demultiplexing
-3
-’0
Or
is demonstrated up to 2OOGbitis with <0.5dB polarisation
I
1
0-1
0-2
0.3
modulation index mi
1594/2)
Fig. 2 CCNR against modulation index f o r different excess loss, Le
0
changes from OdB to 20dB with 2dB steps
external modulation, ....... direct modulation
~
Beyond this optimum modulation index value the CNR decreases
due to CT$. At large modulation index value CNR is decided by
oTd irrespective of the received optical power level. The effect of
local oscillator power is studied in Fig. 3. For both external and
direct modulation systems CNR will increase first when local
oscillator power is increased then it will follow the value change of
a Bessel function, decrease first then increase again. With Le =
15dB, a carrierlnoise ratio of 17dB can be easily achieved with a
local oscillator power of SdBm.
dependency based on four-wave mixing in depolarised TE and
TM modes in a travelling-wavesemiconductorlaser amplifier.
Introduction: All-optical time-division demultiplexing is one of the
most fundamental technologies in realising future Gbitls all-optical networks. Among the requirements for all-optical demultiplexers, polarisation-independent (PI) operation [1, 21 is a vital one
because they usually handle optical signals whose polarisation varies rapidly after transmission of long fibres. In this Letter we propose a simple PI all-optical TDM demultipiexer that utilises the
ultrafast four-wave mixing (FWM) process [3] of both TE and TM
modes in a semiconductor laser amplifier (SLA) in which input
pump and signal pulses are deliberately depolarised to suppress
polarisation coupling in an SLA; as a result, stable error-free PI
operation is achieved up to 200Gbitis with i0.5dB polarisation
dependency.
nm
I
I
100-200GbiVs
0
& -5-
1
t
.I
L
.
0
-1 5 I
I
-10
-5
0
5
10
15
local oscillator power,dBm
20
[594/jl
Time, ps
external modulation. ....... direct modulation
~
Conclusion: The study in this Letter suggests that an optical mixer
can be used for the SCM system. The main advantage is that a
narrowband tuned optical receiver instead of wideband optical
receiver can be used. Variation of CNR with modulation index
and LO power level was studied. The results suggest that a
good system performance can be achieved with practical system
parameters.
0 IEE 1996
4 March I996
Electvonics Letters Online No: I9960562
LU Chao (School of Electronic Engineering, Nanyang Technological
University, Nanyang Avenue, Singapore 2263, Singapore)
References
and OLSHANSKY, R.: ‘Multichannel coherent FSK
experiments using subcarrier multiplexing techniques’, J. Lightwave
Technol., 1990, LT-8, pp. 406415
2 GREENHALGH, P.A., ABEL, R.D., and DAVIES, P.A.: ‘Optical prefiltering
in subcarrier systems’, Electron Lett., 1992, 28, pp. 1850-1852
3 GOPALAKRISHNAN, G K.,
BURNS, w.K.,
and BULMER, c.H.:
‘Microwave-optical mixing in LiNbO, modulators’, IEEE Trans.,
1993, MTT-41, pp. 2383-2391
1
GROSS, R.,
840
Time, ps
Fig. 1 Experimental setup of polarisation-independent all-optical demultiplexer with 100 and 200Gbit/s input signals
MOD: modulator, SOP: state of polarisation, PI: polarisation-independent, PM: polarisation-maintaining, SLA: semiconductor laser
Fig. 3 Carrierhoise ratio against LO power
amplifier
Experiment and results: Fig. 1 shows the experimental setup of the
proposed PI all-optical demultiplexer. The pump and signal
sources were both 6.3 GHz actively mode-locked Er3+-dopedfibre
ring lasers (ML-EDFRL) outputting 1545.9nm, 4.2ps and
1552.7nm, 3.7ps transform-limited pulses with a time-bandwidth
product of 0.38 and 0.37, respectively, for 100GbiUs operation.
For 200GbiVs operation, the pump and signal sources were a
supercontinuum (SC) pulse source [4] that generated 1543.5nm,
1.7ps and 1553.lnm, 1.7ps transform-limited pulses with timebandwidth products of 0.33 and 0.32, respectively. The 6.3GHz
signal pulses were 16 x (or 32 X) time-division multiplexed to
100GbiVs (or 20OGbiUs) after modulation by a LiNbO, intensity
modulator (2I5-l, PRBS). 100GbiUs and 200Gbith input signal
waveforms observed with a newly developed optical sampling
scope with a 0.5ps temporal resolution [5] are also shown in Fig.
1. Both pump and signal pulses were independently amplified and
input to an SLA through a polarisation-maintaining(PM) WDM
multiplexer with pump polarisation at an angle (normally 45
degrees) with respect to TE (TM) polarisation of the SLA to excite
FWM with equal FWM efficiency in both modes. The input pump
and signal average optical powers were 9.4dBm and 2.1 dBm for
100Gbitls operation, and 7.3dBm and 2.3dBm for 200GbiUs oper-
ELECTRONICS LETTERS
25th April 1996
Vol. 32
No. 9